An integrated circuit or chip must have input/output (I/O) connections in order to function as an electronic circuit. Many of these chips require more than 200 I/O connections. For these high count chips, it has been shown advantageous to bring out the I/O connections to land sites arranged in an array pattern on the bottom face of the substrate. Solder balls are attached to these land sites. These solder balls, in turn, provide the I/O connections external to the chip. (IC packages utilizing such solder balls are described in the above-identified related application.)
A technique in which the solder balls are placed on the land sites is well known in the art. FIG. 1 shows a schematic cross sectional view of one such prior art apparatus. Only the dispensing portion 10 of the apparatus is shown. As shown, the dispensing portion 10 has a small chamber 14 which has an upper horizontal wall 13 and a lower horizontal wall 15. The upper wall 13 has an inlet port 12 which permits a positive or negative pressure to be applied to chamber 14. The lower wall 15 has an array of outlet ports 16 bored therethrough. In the example shown in FIG. 2, which is a cross sectional view of the dispenser portion 10 along line 2--2 shown in FIG. 1, the lower wall 15 has a 15.times.15 array pattern of 15 rows and 15 columns of outlet ports, or a total of 225 outlet ports.
In operation, the dispensing portion 10 is placed into a source of solder balls. Generally, the solder balls are of equal size. A negative air pressure is then applied at inlet port 12, thereby creating a partial vacuum within the chamber 14 and suction forces at the outlet ports 16. Due to the suction forces, solder balls (not shown) attach themselves to the outlet ports 16. In the ideal situation, one solder ball should be attached to every outlet port 16 without being jammed into the ports 16.
The dispensing portion 10 is then placed immediately above and aligned with the land sites of the substrate. A positive air pressure is then applied at inlet port 12, thereby releasing the solder balls from the outlet ports and depositing them onto the land sites. Again in the ideal situation, one solder ball should be released from each outlet port 16.
The dispensing portion 10, however, has several shortcomings. The flow of air in the chamber 14 is not uniform during either transient or steady state periods of pressure applications at the inlet port 12. This is due to the directionality of air flow in the chamber 14, where more air tends to pass at locations near the inlet port 12 and less air passes at locations remote from the inlet port 12.
In fact, if one was to examine the pressure within the chamber 14, it would be seen that a pressure gradient exists across the lower wall 15 of the chamber 14. The maximum pressure magnitude would be found at the outlet ports 16 located adjacent to the inlet port 12. The pressure magnitude drops away sharply as one moves toward the outlet ports 16 located furthest away from the inlet port. Furthermore, the air flow velocity is at a maximum through the outlet ports 16 adjacent to the inlet port 12 and at a minimum through the outlet ports 16 remotely located from the inlet port 12.
Due to the aforementioned pressure gradient, there is incomplete pick up and release of solder balls. During the pick up cycle, when negative pressure is applied to the inlet port 12, the vacuum created within the remote outlet ports may be too weak to pick up the solder balls. If the inlet vacuum pressure is increased, the solder balls at the adjacent outlet ports may become jammed within the outlet ports. This, in turn, creates difficulties during the solder ball release cycle.
What has been described thus far is the undesirable effect created when the dispensing portion 10 is used to pick up and place solder balls. It is also possible to use the dispensing portion 10 as an epoxy dispenser for attaching a die to a substrate and it has been used as such. In the latter case, epoxy fluid is transmitted into the chamber 14 by way of the inlet port 12 and out of the chamber 14 by way of the outlet ports 16.
It will be understood that the undesirable effects aforementioned with respect to air flow within chamber 14 are also applicable when the device is used as an epoxy dispenser.
It is, therefore, an object of the present invention to overcome the disadvantages and limitations of prior apparatus by providing a new and improved dispenser.
Another object of this invention is to provide an apparatus for uniformly picking up and depositing solder balls by eliminating the pressure gradients at the lower wall of the chamber.
Yet another object of this invention is to provide an apparatus for uniformly depositing epoxy onto a substrate.